Bone marrow mesenchymal stem cells (BMMSCs) are hierarchical postnatal stem cells capable of self-renewing and differentiating into osteoblasts, chondrocytes, adipocytes, and neural cells (Bianco et al., 2001; Friedenstein et al., 1974; Owen et al., 1988; Pittenger et al., 1999; Prockop et al., 1997).
Due to the heterogeneity of the BMMSCs, there is no single, unique marker allowing for BMMSC isolation, rather an array of cell molecules are utilized to profile BMMSCs. It is widely accepted that BMMSCs express SH2 (CD105), SH3/SH4 (CD73), integrin β1 (CD29), CD44, Thy-1 (CD90), CD71, vascular cell adhesion molecule-1 (CD106), activated leukocyte cell adhesion molecule (CD166), STRO-1, GD2, melanoma cell adhesion molecule (CD146), Octamer-4 (Oct4), and stage-specific embryonic antigen-4 (SSEA4) (Conget et al., 1999; Galmiche et al., 1993; Gronthos et al., 2003; Haynesworth et al., 1992; Martinez et al., 2007; Pittenger et al., 1999; Sacchetti et al., 2007; Shi et al., 2003; Simmons et al., 1991; Sordi et al., 2005). It is generally believed that BMMSCs are negative for hematopoietic cell markers such as CD14 and CD34 with a very low level of telomerase activity (Conget et al., 1999; Covas et al., 2008; Galmiche et al., 1993; Haynesworth et al., 1992; Martinez et al., 2007; Pittenger et al., 1995; Sacchetti et al., 2008; Shi et al., 2002, 2003; Sordi et al., 2005). Recent studies have implied that mouse BMMSCs might express the hematopoietic surface molecules, CD45 (Chen et al., 2007) and CD34 (Copland et al., 2008).
BMMSCs are considered to be progenitors of osteoblasts with the capacity to regenerate bone and marrow components in vivo. These findings have led to extensive studies using BMMSCs for mineralized tissue engineering. The clinical evidence appears to support the notion that BMMSC implantation is able to improve cell-based skeletal tissue regeneration (Kwan et al., 2008; Panetta et al., 2009). Recently, evidence has accumulated that BMMSCs produce a variety of cytokines and display profound immunomodulatory properties (Nauta et al., 2007; Uccelli et al., 2007, 2008), perhaps by inhibiting the proliferation and function of several major immune cells such as natural killer (NK) cells, dendritic cells, T and B lymphocytes (Aggarwal and Pittenger, 2005; Nauta et al., 2007; Uccelli et al., 2007, 2008). These unique properties make BMMSCs of great interest for clinical applications in treating immune disorders (Nauta and Fibbe, 2007; Bernardo et al., 2009).
BMMSCs are thought to be derived from bone marrow stromal compartment, initially appearing as adherent, single colony clusters (colony-forming unit-fibroblasts [CFU-F]), and subsequently proliferating on culture dishes (Friedenstein et al., 1980). Adherent BMMSCs are able to proliferate and undergo osteogenic differentiation, providing the first evidence of CFU-F as precursors for osteoblastic lineage (Friedenstein et al., 1980). For over 40 years, the adherent CFU-F assay has been used as an effective approach to identify and select BMMSCs. To date, the CFU-F assay has been considered to be one of the gold standards for BMMSC isolation and expansion (Clarke et al., 1989; Friedenstein et al., 1970).